Zill Lab NSF Research

RESEARCH PROJECT: TUNING POSTURE AND LOCOMOTION TO BODY LOAD: COMMON MECHANISMS OF LOAD SENSING AND DISCRETE CHANGES IN MOTOR ACTIVITIES IN WALKING IN COCKROACHES. L.A. Quimby; S.N. Zill*. Dept. Anat., JC Edwards Sch. Med., Marshall Univ., Huntington, WV, USA

ABSTRACT: Forces needed to support body weight are generated by the concerted action of legs in stance. These forces continuously vary in individual limbs when walking. We are studying the effects of changes in body load that are imposed via magnets attached to the thorax of freely moving cockroaches. Discharges of tibial campaniform sensilla, receptors that encode forces in the limb via exoskeletal strains, are monitored neurographically. Activities of leg muscles are recorded and animals are videotaped. Data are now being obtained in each of the serially homologous legs. Our results to date indicate that there are common mechanisms for support of body load in all legs. In posture, load increments can elicit vigorous firing of proximal sensilla and the trochanteral extensor muscle of the front, middle or hindlegs. This pattern is consistent with an interjoint reflex that can be elicited by stimulation of proximal receptors in each leg. During walking, proximal sensilla in all limbs are active in stance while distal receptors discharge prior to swing. Our recent studies have shown that sensory activities in the front and middle legs persist in walking with body weight supported over an oiled glass plate. Thus, all groups of sensilla are apparently responsive to both load and muscle forces. In walking of freely moving animals, tonic increases in body load (approximately 30% body weight) produce slower walking rates but with similar temporal parameters within the step cycle. These loads elicit specific changes in extensor activities in the front and middle legs. Extensor bursts are initiated during swing and continue through most of stance but loads produce increased firing only after foot placement. These data suggest that activities of muscles providing support in slow walking are tuned by mechanisms that occur during the stance phase, when discharges of receptors monitoring forces are maximal.
Support Contributed By: NSF Grant IBN-0235997

DETECTION OF BODY LOAD WHEN STANDING: Many studies support the idea that the nervous system can generate postural adjustments to changes in load using information from a variety of sensory modalities. A number of types of receptors can detect loading of legs, including sense organs that directly encode forces, such as muscle tendons organs or receptors of the sole of the feet. Other types of sense organs can signal the effects of forces on legs through changes in body position, muscle length or joint movements (muscle spindles, joint angle receptors).

MODEL OF FORCE DETECTION IN COCKROACH WALKING: We have studied load detection in insects, as sense organs that signal the effects of forces acting upon the limbs (campaniform sensilla) can be recorded in freely moving animals. In posture, proximal sensilla of the front and hind legs can encode body weight. In walking, proximal sensilla fire at stance onset and continue through midstance; distal sensilla fire short bursts at the end of stance when the leg is unloaded. Our working hypothesis is that sensory discharges may result from different sources at different times in stance: firing early in stance can result from forces generated by contractions of muscles that press the leg as a lever against the substrate. Force feedback later in stance may reflect the use of the leg as a supportive strut when body weight is placed upon the limb.

SPECIFIC GOALS:

1- Test postural responses to changes in body load in all legs.

2- Record sensory activities during walking in front and middle legs

3- Test effects of changes in load during walking

a) Decreased body load in walking while supported over oiled glass

b) Tonic increases in load upon leg movements, muscle and sensory activities

RESULTS

EACH LEG PROVIDES SUPPORT OF BODY LOAD: In the cockroach, the homologous pairs of legs are similar in segmental structure and intrinsic musculature but differ in their orientation, size and functions in propulsion. In each leg the intrinsic joints (coxo-trochanteral, femoro-tibial joints) operate in approximately the same plane. Measurements of ground reaction forces have shown that each of the legs can generate forces in support of body weight but differ in their contribution to propulsion. The large hind legs generate propulsive forces while the small front legs brake forward motion in rapid running. The middle legs, intermediate in size, produce both braking and propulsive forces in stance. These forces are reflected in the orientations of the legs in posture and walking. The hind legs are the most horizontal in orientation, while both the front legs and middle legs are more vertical. We have measured the positions of the legs and the center of mass in walking animals. The figures below show the movement of the center of mass relative to the position of the foot (tarsus) during stance for each of the legs. The front leg is anterior to the center of mass at the start of stance but is nearly beneath it by stance end. The hind leg is consistently far behind the center of mass and only approaches it at stance onset. In contrast, the middle leg is directly below the center of mass in mid stance. Thus, intrinsic muscles of the leg should be most effective in generating support of body load in the front and middle legs.

RESPONSE PROPERTIES, REFLEX EFFECTS AND ACTIVATION IN FREELY STANDING ANIMALS: CONSISTENT EXCITATION OF PROXIMAL SENSILLA AND TROCHANTERAL EXTENSOR: We have characterized the structure, responses and reflex effects of the tibial campaniform sensilla of the middle legs in restrained preparations. Units were identified by ablating their cuticular caps, which eliminated responses to imposed force, and aided by use of paired recordings, which allowed for measurements of afferent conduction velocity delay. Application of bending forces, applied using rapid half sine waves, produced short latency excitation of proximal sensilla and trochanteral extensor motoneuron. Decreases in force activated the distal receptors, as has been found in other legs. Use of sinusoidal waveforms showed that the reflex could be elicited over a wide range of frequencies. We have also begun testing the effects of resisted muscle contractions upon sensillum discharges. Bursts in the hind leg trochanteral extensor could produce prolonged activation of proximal sensilla, when the leg was fixed with the femorotibial joint extended to minimize effects of tibial flexor discharges. Thus, sensilla respond to both external and muscle generated forces.

Application of load increases via a magnet and coil in freely standing animals also produced strong excitation of proximal sensilla and the trochanteral extensor motoneuron (Ds). Proximal firing increased following the onset of force application at very short latency and reached a maximum before the peak of body displacement (measured from video images). Thus, proximal receptors provide a rapid signal that loads are increasing, even prior to substantial changes in body position. The peak of extensor firing also occurred early in a burst and the firing rate did not follow the body position but was coincident with the maximum velocity of displacement. In addition, both sensory and motor firing could be modulated by slow changes in load using sine waves and also showed discharges proportional to the amplitude of the applied force. Thus, in the cockroach, information from force receptors can potentially compliment effects of sense organs encoding kinematic parameters to aid adjustments to changes in body load.

SIMILAR PATTERNS OF SENSORY DISCHARGE OCCUR IN WALKING IN EACH LEG: The tibial sensilla of the front and middle legs fired during the stance phase of walking in recordings taken from freely moving animals. Proximal receptors began firing at stance onset and continued through the first two thirds of the stance phase while distal sensilla fired bursts late in stance, prior to leg lifting in swing. Proximal discharges in these legs were at much higher frequencies than seen in the hind leg and showed a general dependence upon the rate of walking. Distal bursts were often more prolonged than the homologous metathoracic receptors. These findings were unexpected, given the diverse functions these legs fulfill in propulsion, but are consistent with the sensitivities to body load shown by sensilla of all legs.

SENSORY DISCHARGES IN FRONT AND MIDDLE LEGS PERSIST IN WALKING WITH BODY WEIGHT SUPPORTED: To evaluate the contribution of body load and muscle generated forces to sensory feedback, we recorded activities when animals walked while suspended above an oiled glass plate. In these preparations, body weight is largely supported but all forces generated by leg muscles in the vertical plane are resisted by the rigid bar supporting the animal. The pattern of alternating discharges of proximal and distal receptors persisted in these tests and the highest firing rates occurred in the middle legs, which are located immediately below the resisting support. These findings imply that a major determinant of afferent feedback is the force generated by leg muscles. In walking, this feedback could readily serve to produce forces sufficient to support the body load and unload legs already in stance

EFFECTS OF TONIC INCREASES IN LOAD ON LEG POSITION AND MOVEMENT IN WALKING: In these experiments, load was tonically increased by adding small magnets to the thorax. Sequences of free walking were videotaped at high speed and the times and positions of the legs at the start and end of stance were digitized. Tonic increases in load of 30% body weight regularly produced small decreases in the rate of walking. To determine the effects of this level of loading on leg movements, we tested the effects of adding weights to animals that had no recording wires. These animals also showed a small decrease in the duration of the swing phase but no regular changes in stride length. Some animals also showed shifts in the range of leg movements (anterior and posterior end points) that moved the legs closer to the center of mass. We therefore utilized this level of load in tests in which motor and sensory activities were recorded.

TONIC INCREASES IN LOAD PRODUCE DISCRETE EFFECTS UPON MOTOR ACTIVITIES IN WALKING: Recordings of the trochanteral extensor muscle in freely moving animals showed similar patterns of activities in each of the homologous legs. Extensor bursts are initiated during the swing phase and continue through most of stance. Extensor firing in swing is most prolonged in the front legs, where it serves to lower the leg toward the substrate in foot placement. Tonic increases in load produced specific elevations in extensor firing: motoneuron frequencies were increased after stance onset but not elevated during swing. Similar results were obtained at all rates of walking. Thus, animals do not apparently change extensor motoneuron activities in anticipation of load but only after load is detected.

TONIC INCREASES IN LOAD ELEVATE PROXIMAL FIRING IN STANCE: In these experiments, sequences were recorded in which animals first walked with no added weight, and then had additional applied weight to the thorax. These histograms compare proximal sensillum discharges in stance in walking at the same mean step frequency from animals with and without the 30% additional body weight. Sensillum firing is elevated during the first two thirds of stance with the additional weight. Further tests are planned in which loads are increased at higher levels or imposed suddenly during the stance phase using the magnetic coil. However, these results are consistent with the hypothesis that sensillum firing can reflect body load in walking.

CONCLUSIONS

1) In posture, activation of the proximal sensilla and trochanteral extensor muscles occur to imposed increases in body load in each of the animals serially homologous legs. This activation pattern is consistent with a reflex that can be elicited in each of the limbs. This sensory motor connection could therefore contribute to the support of body load.

2) Responses from middle and front legs were substantially higher than those obtained from hind legs. These responses are consistent with the more vertical orientation of the leg axis producing forces to counter the gravitational vector.

3) In walking, similar patterns of alternating activation of proximal and distal sensilla occur during the stance phase in all legs.

4) Sensillum discharges persist in walking over an oiled glass surface when body weight is supported by a rigid bar. We attribute these discharges to muscle generated forces which press the leg to the substrate at the start of the stance phase. These discharges are more prolonged in the front and middle legs, consistent with their orientation.

5) Animals were readily able to walk when tonic loads were added to the thorax. Walking generally occurred at slower rates, reflecting an increase in stance phase duration. These loads also produced small but statistically significant decreases in swing duration and shifts in the range of leg movement, but no large changes in kinematic parameters.

6) Recordings of the trochanteral extensor of the front and middle legs showed consistent and highly specific changes in firing frequency. These muscles are active both in swing and stance to lower the leg and press it against the substrate. Tonic loads increased activity in stance but not in swing. Thus, the changes in motoneuron firing were reactive and not predictive.

7) We are currently recording activities of sensilla during walking at various loading levels. Our results to date show that the sensillum firing is significantly increased in the first third of stance. These data support the hypothesis that receptors that monitor forces in the limbs can aid in enhancing and adjusting muscle activities to the level of body load.

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